Simplified system for feeding over-fire air to a heater for a low NOx emission

ABSTRACT

A system for feeding over-fire air to a combustion chamber ( 10, 24, 42 ) of a steam or hot water generation plant of low-medium capacity, using solid, liquid or gaseous fuel or a combination thereof, comprising a main combustion air duct ( 5 ) supplied to a burner unit ( 11, 25, 43 ) in the combustion chamber and an over-fire air duct ( 12, 27, 44 ), which extends within said combustion chambers and is provided at its end with nozzle means ( 17, 27, 47 ) for said over-fire air.

DESCRIPTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a simplified system for feedingover-fire air (O.F.A.) designed to be installed on industrial heaters oflow-medium capacity, namely lower than 25 MW, apart from the fuel used,with the purpose of keeping the pollutant emissions, in particularnitrogen oxides, as low as possible.

[0003] 2. Description of the Prior Art

[0004] A widespread technique to reduce the production of nitrogen oxide(NO_(x)) in the combustion processes occurring in industrial plants andelectric power generation plants is that of the so-called stepcombustion, which consists in suitably dosing the air and fuel withinthe combustion system, in such a way to form a fuel rich zone in whichthe fuel pyrolisis processes occur and NO_(x) reduction processes areactivated. This zone is followed by a fuel lean zone, wherein thecombustion reaction is completed by mixing the remaining part of thecombustion air with the fuel. In practice, by reducing the oxygenavailability in the primary flame zone the speed of formation ofnitrogen oxides both from the fixation of the atmospheric nitrogen(thermal NO_(x)) and from the oxidation of the nitrogen contained in thefuel (chemical NO_(x)) is inhibited. Due to the reduced presence of theoxygen the nitrogen present in the fuel is forced to recombine withother nitrogen, thus forming molecular nitrogen, N₂, instead of nitrogenoxide. Similarly, due to the lower temperatures which are reached, lowercombustion temperature peaks occur thereby reducing the formation of thethermal NO_(x).

[0005] A way of putting into practice the step combustion technique isthat described in European Patent No. 0452608, according to whichcombustion air is subdivided into three streams, namely primary air,secondary air and tertiary air, which are supplied to the combustionchamber near the burner outlet coaxially to the fuel inlet. Another wayof putting into practice the step combustion is that according to theOFA technique which consists in diverting a portion of the combustionair from the burners to over-fire air ports placed over-fire on thefurnace wall downstream of the burner and introducing the portion ofcombustion air into the furnace through these ports. The amount ofdiverted combustion air is controlled so that the ignition combustion ofthe fuel occurs at sub-stoichiometric condition to create a reducingatmosphere which minimizes the formation of nitrous oxides. Theover-fire air supply system varies according to the embodiments andoften comprises air injectors, swirling vanes, separate blowers andother associated equipment, resulting in installations that are complexand expensive.

[0006] In order to comply with the more and more stringent regulationsregarding the reduction of pollutant emissions and in particular nitrousoxide (NO_(x)) emissions, the retrofitting of the existing plants isnecessary. This may involve structural modification to the furnace oradditional exhaust gas treatment plants (for example, the so-calledDeNO_(x) plants). In the case of plants for the production of hot wateror steam of low-medium capacity, the above mentioned alternatives areboth unfeasible both for technical and cost reasons.

[0007] In particular, if a step combustion according to the OFAtechnique would be used to reduce the NO_(x) emissions, openings wouldhave to be formed in the part under pressure of the furnace to createthe over-fire air supplying ports, which would give rise to many seriousproblems.

SUMMARY OF THE INVENTION

[0008] The object of the present invention is to provide a system forsupplying the over-fire air to a furnace of low-medium capacity for hotwater or steam generation characterized by a low construction cost, inparticular if applied to the retrofitting of existing plants.

[0009] According to the invention the system for supplying the over-fireair to a furnace for the generation of hot water or steam comprises aover-fire air duct extending within the combustion chamber in such a wayto provide the over-fire air injection from a rear wall of the furnacewith respect to that where the burner is placed, or sidewise withrespect to the same wall through an existing opening, for example amanhole, or a dedicated opening, formed in a not pressurized part of thefurnace, for example through the refractory material.

[0010] If the over-fire air is injected from the rear wall, theinjection is performed through a suitable nozzle which can be coaxial tothe burner, if the furnace is equipped with only one burner, or coaxialto the burner assembly, if the furnace is equipped with more than oneburner. In the case of a side injection, the nozzles for supplying theover-fire air are arranged near the sidewalls of the furnace, generallynear the floor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Features and advantages of the over-fire air supplying system forindustrial furnaces according to the present invention will becomeapparent from the following description of exemplifying, non-limitingembodiments thereof made with reference to the attached drawingswherein:

[0012]FIG. 1 schematically shows a furnace with relevant supplyingsystem of the main combustion air and of the over-fire air according tothe prior art;

[0013]FIG. 2 shows a system for rear injecting the over-fire airaccording to the invention;

[0014]FIG. 3 shows a system for sidewise injecting the over-fire airaccording to the invention;

[0015]FIGS. 4 and 5 show two different nozzles for injecting over-fireair in a rear injection system;

[0016]FIGS. 6 and 7 show two different supplying systems for sidewiseinjecting the over-fire air according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0017] The diagram shown in FIG. 1 represents the conventional solutionfor supplying the combustion air to an industrial furnace, generallyindicated at 1, wherein the combustion air is divided into two streams,one of which is supplied to the burners together with the fuel and theother is directly supplied to the upper part of the furnace as over-fireair. Burners 2, arranged in one or more arrays, are connected to therelevant conduits 3 and 4 for feeding the fuel and, respectively, forfeeding the main combustion air. Ducts 4 branch from a main air header 5coming from a heat recovery unit 6, for example a Ljungstrom, incountercurrent to the combustion exhaust gas 7 directed to the stack.The combustion air is supplied by blower means 8 located upstream of thefurnace. A certain amount of the combustion air is sucked by a fan 9from combustion air duct 5 downstream of heat recovery unit 6 to besupplied to the upper part of the furnace, indicated at 1 a, to completethe combustion. The inlet of the over-fire air in the combustion chamberis achieved through a series of ports formed on the furnace wall.

[0018] The overall flow rate of the combustion air, the flowrate of theair supplied to the single burners, the flowrate of air drawn asover-fire air as well as that of the air admitted through the singlesupply ports are controllable within large ranges by means of suitableair locks as schematically shown in FIG. 1.

[0019] In FIG. 2 there is shown, in a longitudinal cross-section, asystem for the rear injection of the over-fire air according to thepresent invention. With reference to this figure, there has beenindicated at 10 a combustion chamber and generally at 11 a burner or aburner assembly, for example a group of two or four burners. Anover-fire air injection device 12 extends within the combustion chambercoaxially to the burner from a nozzle 13 located on the rear wall of thefurnace through an adjustment device, comprising a flange 14, acounterflance 15 and a gasket 16, through which the protrusion ofinjection device 12 within the combustion chamber can be regulated so asto allow the axial movement of an injection nozzle 17 mounted at thefree end of injection device 12. Injection device 12 is lined with alayer of insulating material 18. At the outer side of combustion chamber10 injection device 12 is connected to an air inlet 19 possibly equippedwith a control air lock 20 and a closure flange 21 with a handle 22extending therefrom for clamping a rod for moving nozzle 17, in the caseof mobile nozzles.

[0020] A system for over-fire air supply with side injection is shown inFIG. 3. In this figure the combustion chamber has been indicatedgenerally at 24 and 25 indicates a generic burner or burner assembly.Located at the front wall of the furnace, in particular below burner 25,is a nozzle 26 within which a side over-fire air duct 27 extending incombustion chamber 24 is placed. Duct 27 ends with an injection duct 28transversally arranged to the combustion air flow and equipped with aplurality of tubular nozzles 29 having circular, square or rectangularcross-section. Outside of combustion chamber 24 duct 27 is connected toan air inlet 30 possibly equipped with an adjustable air lock 31 for theover-fire air.

[0021] As a function of the design needs injection duct 28 may extendalso along the side walls, taking a U-shaped configuration and theinjection nozzles 29 may be replaced by a narrow slit extending alonginjection duct 28 in such a way to supply the over-fire air to thefurnace in the form of a continuous laminar barrier instead of discretejets.

[0022] Nozzles of the conventional type, which can be used for supplyingthe over-fire air to the combustion chamber in the case of rearinjection of the over-fire air, are shown in FIGS. 4 and 5. The nozzleshown in FIG. 4 provides a continuous radial outlet for the over-fireair and comprises a bottom plate 32 lined with insulating material 33from which a connection system 34 of rod 23 for moving the nozzleextends. The nozzle is equipped with a plurality of fixing plates 35 formechanical strengthening and a plurality of blades 36 arranged outsideof the nozzle to convey the over-fire air.

[0023] The nozzle shown in FIG. 5 provides a radial outlet in the formof jets for the over-fire air and comprises a bottom plate 37 lined withinsulating material 38 from which a connection system 39 of rod 23 formoving the nozzle extends. The nozzle is provided with a plurality offixing plates 40 for mechanical strengthening and a plurality ofdiverting blades 41 arranged in the outer side of the nozzle to directthe over-fire air outlet.

[0024]FIGS. 6 and 7 show other side injection modes for the over-fireair as in the case of FIG. 3, but with an air supply coaxial to theburner and in particular rear coaxial supply (FIG. 6) and front coaxialsupply (FIG. 7). In the case of FIG. 6, in which 42 and 43 generallyindicate a combustion chamber and, respectively, a burner or burnerassembly, an over-fire air supply duct 44 is introduced in the furnacethrough a manhole 45 and then extends vertically down to the floor andfinally runs longitudinally thereon, in particular over the tube bundle.Duct 44 ends with a transverse injection duct 26 bearing tubularinjection nozzles 47. The solution of FIG. 7, on the other hand, can beadopted when a manhole or another opening in a suitable position areunavailable or when the solution of FIG. 3 cannot be adopted. In thiscase the air supply duct is formed around burner 43 and extendstherefrom within the combustion chamber in the same way as shown in FIG.6.

[0025] Advantageously, in order to further reduce the smoke point of theexhaust gas discharged from the stack, a part of the exhaust gas comingfrom the furnace, in a percentage not higher than 15%, may berecirculated. It can be injected in the overall combustion air stream,in the air supplied to the burners only, in the over-fire air only oreven in one of the burner conduits.

[0026] With respect to the systems of supplying and injecting over-fireair according to the prior art, the system of the invention allows thesame typical step combustion to be carried out with the same level ofreduction of NO_(x) (up to about 200 mg/Nm³) by means of a structurallysimple solution for low-medium capacity steam and hot water generatorsand in particular it makes the OFA technique applicable to existinggenerators of the above mentioned type without requiring modificationsof the pressurized parts of the furnace, as existing accesses to thecombustion chamber may be used.

[0027] Variations and modifications can be brought to the over-fire airsupply system according to the present invention, without departing fromthe scope of the invention as set forth in the appended claims.

1. A system for feeding over-fire air to a combustion chamber (10, 24,42) of a steam or hot water generation plant of low-medium capacity,using solid, liquid or gaseous fuel or a combination thereof, comprisinga main combustion air duct (5) supplied to a burner unit (11, 25, 43) insaid combustion chamber and an over-fire air duct (12, 27, 44),characterized in that said over-fire air duct (12, 27, 44) extendswithin said combustion chamber and is provided at its end with nozzlemeans (17, 27, 47) for said over-fire air.
 2. Over-fire air feedingsystem according to claim 1, wherein said over-fire air duct (12)extends within said combustion chamber coaxially to said burner unit(11) from a wall opposite thereto and ends with a nozzle (17) forradially injecting the over-fire air in said combustion chamber. 3.Over-fire air feeding system according to claim 2, wherein said nozzle(17) in axially sliding.
 4. Over-fire air feeding system according toclaim 2, wherein said radial over-fire air outlet is continuous. 5.Over-fire air feeding system according to claim 2, wherein said radialover-fire air outlet are air jets.
 6. Over-fire air feeding systemaccording to claim 2, wherein said over-fire air duct (12) adjustablyextends within said combustion chamber (10).
 7. Over-fire air feedingsystem according to claim 1, wherein said nozzle means (29, 47) for theover-fire air are located close to the side walls of said combustionchamber (24, 42) and are arranged on an injection duct (28, 46)crosswise extending within said combustion chamber and communicatingwith said over-fire air duct (27, 44).
 8. Over-fire air feeding systemaccording to claim 7, wherein said nozzle means (29, 47) are equallyspaced tubular nozzles.
 9. Over-fire feeding system according to claim7, wherein said nozzle means are formed by a continuous slit along saidinjection duct (28, 46).
 10. Over-fire air feeding system according toclaim 7, wherein said over-fire air duct (27, 44) is entered saidcombustion chamber (24, 42) through a manhole (45).
 11. Over-fire airfeeding system according to claim 7, wherein said over-fire air duct(27, 44) is entered said combustion chamber (24, 42) through an openingformed in a not-pressurized part of the generator.
 12. Over-fire airfeeding system according to claim 7, wherein said over-fire air duct(44) is entered said combustion chamber (42) coaxially to said burnerunit (43).
 13. Over-fire air feeding system according to claim 1,wherein a portion of the exhaust gas is added either to the overallcombustion air flow or to the air supplied to all or some of the burnersor to the over-fire air flow only.